The present invention relates to a control system for controlling a solenoid-operated valve in a hydraulic circuit of an automatic transmission for a motor vehicle, and more particularly to the system for correcting a pressure override of a relief valve for the solenoid-operated valve.
The control system for a continuously variable transmission (CVT) comprises an endless belt running over a primary pulley and a secondary pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid-operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with the hydraulic circuit including a pump for supplying oil to the servo devices, and a pair of these solenoid-operated control valves for controlling secondary pressure and primary pressure in the circuit.
The system employs an electronic control system for controlling the valves of the hydraulic circuit. The solenoid-operated control valve is operated by a proportional solenoid-operated relief valve and controls the secondary pressure in proportion to control current applied to a solenoid.
FIG. 3 shows characteristics of the secondary pressure Ps responsive to a rotating speed Nop of the oil pump with solenoid current Is as a parameter. As the rotating speed Nop increases, the secondary pressure Ps increases. The relief valve opens at a predetermined pressure called a desired secondary pressure. Even after a relief valve opens, the secondary pressure Ps increases slightly because a spring provided in the relief valve urges against internal resistance of the valve, and others. The difference between this higher secondary pressure and the desired secondary pressure is called pressure override. The flow rate of the oil through the secondary valve changes in accordance with changes in discharge flow rate of the oil pump, temperature of the oil, and transmission ratio changing speed, and others. The pressure override fluctuates with the change of the flow rate through the secondary valve.
Because the pressure override fluctuates, the actual secondary pressure does not coincide with the desired secondary pressure, causing malfunctioning and the belt slipping. Furthermore, excessive pressure may be applied to a hydraulic cylinder.